Propeller Basics: Torque to Thrust

081-07-01Pablo Asensio Martínez2026-04-011 min

Conversion

The propeller acts as a rotating wing.

  • Resultant Force: Has a forward component (Thrust) and a rotational component (Torque Drag).
  • Engine Torque: Must overcome the Torque Drag to keep the propeller spinning.

Pitch Definitions

  1. Geometric Pitch: The distance the propeller should move forward in one revolution if it were a screw moving through a solid (theoretical).
  2. Effective Pitch: The actual distance it moves forward in one revolution (in air).
  3. Slip: The difference between Geometric and Effective Pitch. $Slip = Geometric - Effective$.
    • Note: Slip is necessary to produce Thrust (creates Angle of Attack). Zero slip = Zero thrust.

Blade Twist (Washout)

The blade angle decreases from the root to the tip.

  • Reason: The rotational velocity ($V_r$) increases towards the tip ($V = \omega r$). To maintain a constant, efficient Angle of Attack (AoA) along the entire blade, the pitch must be coarser at the root and finer at the tip.

Efficiency ($\eta$)

Ratio of Power Output (Thrust Power) to Power Input (Brake Power).

  • $\eta = \frac{Thrust \times TAS}{Brake Power}$
  • Optimal efficiency occurs at a specific Advance Ratio ($J$).